As a uni-directional conducting device, the insulated gate bipolar transistor (IGBT) requires an anti-parallel diode to release the energy stored in the inductive load in a transient turn-off state. Conventionally the anti-parallel diode is externally co-packed with the IGBT. This results in additional BOM (Bill of Material), additional testing time, and increased cost for the separate devices, and associated cost for the package. Moreover the approach of co-packing the IGBT and diode in one package increases the probable yield loss of the module, as failure of any of these devices results in malfunction of the whole part.
Recently the development of reverse-conducting IGBTs (RC-IGBTs), i.e. the combination of the IGBT and the diode in a monolithic chip, has attracted much attention [see e.g. H. Takahashi, A. Yamamoto, S. Aono, T. Minato, T, “1200V reverse conducting IGBT”, ISPSD 2004, pp. 133-136; H. Ruthing, F. Hille, F. J. Niedernostheide, H. J. Schulze, B. Brunner, B, “600 V Reverse Conducting (RC-)IGBT for Drives Applications in Ultra-Thin Wafer Technology”, ISPSD 2007, pp. 89-92; D. Kumar, M. Sweet, K. Vershinin, L. Ngwendson, E. M. S. Narayanan, “RC-TCIGBT: A Reverse Conducting Trench Clustered IGBT”, ISPSD 2007, pp. 161-164; T. Kimmer, J. Oehmen, P. Türkes, S. Voss, “Reverse Conducting IGBT—A new Technology to Increase the Energy Efficiency of Induction Cookers”, PESC 2008, pp. 2284-2287]. A conventional way to realise a RC-IGBT is to have a N+ region embedded in an anode/collector region of the IGBT. The N+ region is shorted by an anode/collector terminal to a P+ anode injector and is also in contact with a N-buffer region or a N-drift region in case of a non-punch-through (NPT) IGBT. This short allows the formation of an anti-parallel diode, which enables current conduction in the IGBT in a reverse mode (current flows from a cathode/emitter terminal to the anode/collector terminal).
Some known RC-IGBT structures are described in the following.
U.S. Pat. No. 5,260,984 discloses a reverse conducting IGBT in which the IGBT and the diode are co-packed in a package. This leads to insulation, chip displacement and wiring issues for the package. In addition, this also results in a parasitic inductance which restrains the switching speed of the IGBT. This document proposes to separate the IGBT and the diode in two portions but in one single chip, with an intermediate non-interference region being provided to avoid latch-up. Conventional anode shorts have been adapted in the IGBT portion for reducing the quantity of minority carriers in the drift region.
U.S. Pat. No. 5,702,961 relates to a reverse conducting IGBT in which the diode is formed between the IGBTs so as to save cost on area. Furthermore, the diode and the IGBT share the same top metal so that there is no need for wiring the anode of the diode and the emitter of the IGBT. The design of the N+ region of the diode is conventional in which the N+ region is in contact with the N+ buffer region.
U.S. Pat. No. 7,112,868 concerns a reverse conducting IGBT which is particularly used where the required current-carrying capacity of the diode is less than that of the IGBT. In the disclosed structure, the N+ region is placed at the high-voltage edge (i.e. termination) using the conventional approach.
U.S. Pat. No. 7,154,145 discloses a reverse conducting IGBT which reduces reverse recovery current and turn-off time of the anti-parallel diode. The document proposes to create P-type side diffusion regions beside the trenches. In the disclosed structure, the area of P-type region is enlarged so that the hole injection efficiency, when the diode turns on, is enhanced. The reverse conducting structure of the disclosed device relies on the conventional approach, i.e. a backside N+ region directly contacting the N-drift region of the IGBT.
Conventional RC-IGBT structures have however several drawbacks. The first is that there is an undesirable snapback characteristic realised when the IGBT is in a forward mode. This snapback characteristic has a negative resistance associated with it which produces harmonics and can lead to undesirable ringing and oscillations in the system. It has been demonstrated that, by reducing the area density of the N+ regions compared to the P+ anode region, it is possible to reduce the snapback characteristic but at the expense of a poorer performance of the anti-parallel diode in the reverse mode (that is the conduction through the IGBT).
Thus there is a need for an efficient RC-IGBT design to address the trade-off between the IGBT performance in the forward mode and in the reverse mode.
It is an object of the present invention to provide a simple and cost effective RC-IGBT design which at least mitigates the above trade-off.